Semiconductor device including via hole

ABSTRACT

A semiconductor device has a semiconductor material substrate a first insulating film formed on the substrate, a first metallic wiring formed partly on the first insulating film, a second insulating film formed on the first insulating film, a second metallic wiring formed partly on the second insulating film and a via hole for electrically connecting the first metallic wiring and the second metallic wiring. The via hole has a lower portion which extends below the level of a top face of the first metallic wiring and is tapered to help keep the via hole electrically insulated from the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having a viahole, and a manufacturing method thereof. The invention particularlyrelates to a semiconductor device having a layout rule, which is that avia hole connecting an upper metallic wiring with a lower metallicwiring extends down along a side of the lower metallic wiring, and amanufacturing method thereof.

2. Discussion of the Background

As a semiconductor device becomes more complex and is scaled downfurther, a multilayer wiring structure is generally adopted as a wiringtechnique of the semiconductor device, and it becomes more important toimprove the vertical alignment between a wiring and a via hole.

FIG. 8 is schematic sectional view showing an example of a semiconductordevice in which the multilayer structure is adopted. In thissemiconductor device, as shown in FIG. 8, a field oxide film 2 and afirst gate electrode 3 are formed on a silicon (Si) substrate 1. Secondgate electrodes 3' are formed on the field oxide film and are used as awiring. An insulating film 4, typically a silicon oxide film, is formedon the first and the second electrodes 3 and 3', the field oxide film 2,and the entire substrate 1. A metallic wiring 5 of aluminum or anothersuitable material, which is a lower wiring, is formed on the insulatingfilm 4. A second insulating film 6, typically a silicon oxide film, isformed on the first wiring 5 and the first insulating film 4. A secondmetallic wiring 8 of aluminum or another suitable material, which is anupper wiring, is formed on the second insulating film 6 in a mannersimilar to forming the first metallic wiring 5. The second metallicwiring 8 is electrically connected with the first metallic wiring 5 byway of the via hole 7.

However, as seen in FIG. 8, a shift in position of a mask pattern forforming the via hole causes the via hole 7 to extend laterally from thefirst metallic wiring 5. Therefore, a short or leakage, between the viahole 7 and the gate electrodes 3' or the silicon substrate 1 may becaused.

There is a method shown in FIG. 9 for addressing the above problem. Asshown in FIG. 9, side wall spacers 9 of an amorphous silicon and thelike, which typically can be etched at a different rate relative to asilicon oxide film used as the second insulating film, are formed atboth sides of the metallic wiring 5, and the width of the first metallicwiring is increased substantially. As a result, it becomes less likelythat a short will develop between the via hole and the gate electrodes3' or the silicon substrate 1 disposed under the first metallic wiring5.

However, when the side wall spacers are formed at the sides of the firstmetallic wiring, the space between two adjacent first metallic wiringsis decreased substantially, and a margin of a step coverage of thesecond insulating film is decreased. Therefore, it can become moredifficult to scale down the semiconductor device. Also, when the sidewall spacers are formed, a step for forming a side wall spacer film anda step for etching back are added after forming the first metallicwiring 5. As a result, manufacturing costs of such devices are higherand a yield rate of such devices is decreased.

On the other hand, as seen in FIG. 10, when a material of the secondmetallic wiring 8 is buried into the via hole 7 using high temperaturesputtering or reflow of metal, the via hole 7 is formed in a taper shapeof approximately 80 degrees in order to improve a covering rate of thematerial of the metallic wiring. In this case, the angle θ of the taperis usually constant.

When the semiconductor device is scaled down, the size of the via holeis smaller, e.g. 0.5 μm, and the requirements for vertical alignmentbetween the via hole and the first metallic wiring are more stringent.If the angle of the taper is approximately 80 degrees, an area in whichthe via hole is contacted with the first metallic wiring becomes stillsmaller. Therefore, a contact resistance between the via hole 7 and thefirst metallic wiring 5 is increased, and it becomes more difficult toachieve high speed operation of the semiconductor device.

Furthermore, Japanese Laid Open Patent 04-167524 discusses a method forcontrolling a taper shape of a via hole. In this method, the via hole isetched by a mixture of CHF₃ and CF₄ gas, and a flow rate of the gas ischanged continuously as the etching proceeds in order to form the viahole in a taper shape. That is, a ratio of CF₄ is changed from 100 into10 and then into 50 gradually so that an upper portion of the via holecan be a taper shape and a lower portion of the via hole can be avertical shape.

However, in this method, as the lower portion of the via hole is formedin a vertical shape, there is concern that the via hole may contact thegate electrode or the silicon substrate. As a result, a short or leakagemay occur between the via hole, the gate electrode and/or the siliconsubstrate. Also, as the upper portion of the via hole is formed in ataper shape, an area in which the via hole contacts with the firstmetallic wiring decreases, thereby increasing the contact resistancebetween the metallic wiring and the via hole. Therefore, using thetechnique discussed in the Japanese patent, can reduce the operationalspeed of the semiconductor device.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductordevice which reduces the likelihood of an electrical leak or anelectrical short between a via hole and a gate electrode or a siliconsubstrate disposed under a metallic wiring.

It is another object of the present invention to provide a semiconductordevice in which a contact resistance between a via hole and a metallicwiring is decreased to permit high operational speeds of thesemiconductor device.

It is another object of the present invention to provide a manufacturingmethod of the above described semiconductor devices.

These and other objects and advantages are achieved by the presentinvention which provides for a semiconductor device comprising asubstrate of semiconductor material, a first insulating film formed onthe substrate, a first metallic wiring formed at least partially on thefirst insulating film, a second insulating film formed on the firstinsulating film, a second metallic wiring formed at least partially onthe second insulating film, and a via hole for connecting the firstmetallic wiring and the second metallic wiring, wherein a lower portionof the via hole extends down under a top face of the first metallicwiring, and is formed in a substantially taper shape to keep the viahole from reaching the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

In describing preferred embodiments of the present invention illustratedin the drawings, specified terminology is employed for the sake ofclarity. However, the invention is not intended to be limited to thespecific terminology so selected, and it is to be understood that eachspecific element includes suitable equivalents.

A more complete appreciation of the invention and attendant advantagesthereof will be readily obtained as the same become better understood byreference to the following detailed description when considered inconnection with the accompanying drawings, wherein:

FIG. 1 is a schematic sectional view illustrating a semiconductor deviceof a first embodiment of the present invention;

FIG. 2 is a schematic sectional view of the embodiment of FIG. 1,illustrating dimensional characteristics of the semiconductor device;

FIG. 3 is a schematic sectional view illustrating a semiconductor deviceof a second embodiment of the present invention;

FIG. 4 is a schematic sectional view greatly enlarged of thesemiconductor device of FIG. 3, illustrating a via hole connecting firstand second wirings of the semiconductor device;

FIG. 5 is a schematic sectional view greatly enlarged of the embodimentof FIG. 3, illustrating dimensional characteristics of the semiconductordevice;

FIG. 6 is a schematic sectional view illustrating a semiconductor deviceof a third embodiment of the present invention;

FIG. 7 is a schematic sectional view greatly enlarged of thesemiconductor device of FIG. 6, illustrating a via hole connecting firstand second wirings;

FIG. 8 is a schematic sectional view illustrating a prior artsemiconductor device which adopts a multilayer wiring structure;

FIG. 9 is a schematic sectional view illustrating a prior artsemiconductor device which adopts a multilayer wiring structure and sidewall spacers; and

FIG. 10 is a schematic sectional view illustrating a prior artsemiconductor device which adopts a multilayer wiring structure having avia hole formed in a taper shape.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the figures, embodiments of this invention will bedescribed. FIGS. 1 and 2 illustrate a first embodiment of a multilayerwiring structure for semiconductor devices. As seen in FIG. 1, thesemiconductor device includes, a field oxide film 2, preferably asilicon oxide film, and a first gate electrode 3 made of polysilicon,which are formed on a substrate 1. The substrate may be formed from anyknown substrate material, such as silicon. Second gate electrodes 3' areformed on the field oxide film 2 and are used as a wiring. A firstinsulating film 4, preferably a silicon oxide film, is formed on thesubstrate 1 and a first metallic wiring 5, which is a lower wiring, isformed on the first insulating film 4. The first metallic wiring 5 ispreferably composed of an aluminum. However, metals and alloys havingsimilar melting point and/or multilayer wiring characteristics are alsocontemplated.

A second insulating film 6 of a silicon oxide is formed on the firstinsulating film 4. The second insulating film 6 can be formed at leastpartially from silicon oxide, silicon nitride and the like. A secondmetallic wiring 8 of the same or similar material and structure as thefirst metallic wiring 5, is formed on the second insulating film 6.

A via hole 7 is formed by plasma etching using a reactive gas in orderto electrically connect the second metallic wiring 8 to the firstmetallic wiring 5. The via hole 7 extends down along a side of the firstmetallic wiring in a layout rule, as seen in FIGS. 1 and 2. In the viahole 7 of this invention, a taper angle of an upper portion of via hole7 is different from that of a lower portion 7a. The taper angle of thelower portion 7a is formed by over-etching and is, for example, 88degrees or less.

At least one portion of the via hole 7 of this invention can be formedin a taper shape by changing the plasma etching parameters in steps whenforming the via hole.

A manufacturing method for forming the via hole 7 is as follows. Asnoted, the preferred technique for creating the via hole is by plasmaetching. Systems for plasma etching are known. An end of an etchingcycle of the via hole 7 is detected by monitoring a luminous intensityof the plasma. To illustrate, when the etching of the upper portion ofthe insulating film 6 over the top face 5a of the first metallic wiring5 has been completed, the amount of etched material is decreased.Therefore, the end of the etching cycle can be detected. Over-etching of30-100 percent is achieved by further executing the etching cycle toform the taper in the via hole. The percentage of over-etching isdependent on, for example, the uniformity of the etching and thedifference of thickness of the second insulating film 6. Thus, as seenin FIG. 1, the via hole 7 extends down along a side of the metallicwiring 5 because the oxide layer 6 is etched deeper along a side ofwiring 5 due to this over-etching.

To reduce or prevent leakage between the via hole 7 and the substrate 1or the gate electrodes 3', the etching conditions are changed when theend of an etching cycle is detected and the taper angle of the lowerportion 7a of the via hole 7 is made narrower, so as to keep the viahole 7 from reaching the substrate or the gate electrode, i.e., to keepthe via hole insulated from the substrate and gate electrode. The changein the taper angle of the via hole 7 can be achieved by, for example,changing an etching pressure, changing a flow rate of an etching gas, orchanging the kind of etching gas used.

Next, referring to FIG. 2, an example of dimensional characteristics forthe semiconductor device will be described. The exemplary semiconductordevice described herein adopts the following layout rule. The width ofthe first metallic wiring is 0.60 μm, the diameter of the via hole 7 is0.60 μm, the thickness of the upper portion of the second insulatingfilm 6 over the top face 5a of the metallic wiring 5 is 0.50 μm, thethickness of the lower portion of the second insulating film 6 under thetop face 5a of the metallic wiring 5 is 0.50 μm, and the width of theportion of the via hole 7 which extends laterally from the firstmetallic wiring is 0.30 μm.

In this semiconductor device, the etching is set to be changed in twosteps so that the angle of the upper portion 7b of via hole 7 can beabout 90 degrees and that of the lower portion 7a of via hole 7 can beabout 75 degrees in a taper shape.

At this time, etching of a bottom face of the via hole 7 extending fromthe first metallic wiring 5 becomes more difficult due to a lie-lageffect as the over-etching process proceeds. Therefore, as the bottomface of via hole 7 becomes narrower, leakage between the via hole 7 andthe silicon substrate 1 or the gate electrodes 3' can be reduced orprevented.

Next, referring to FIGS. 3 and 4, a second embodiment of the multilayerwiring structure for semiconductor devices will be described. In thissecond embodiment, the structure of the semiconductor devices issubstantially similar to the structure of the first embodiment.Therefore, the same or similar elements are identified by the samenumbers and the description of similar features will be omitted for thesake of convenience.

In this second embodiment, shown in FIGS. 3 and 4, the first metallicwiring 5 is tapered at a predefined angle θ₃ which is 88 degrees orless. RF power is controlled and/or a depositional gas, for example,silicon tetrachloride (SiCl₄) or ammonia (NH₃), is used in the plasmaetching cycle when forming the taper in the metallic wiring.

Referring to FIG. 5, an example of dimensional characteristics for thisembodiment of the semiconductor device will be described. In the secondembodiment, the width of the first metallic wiring 5 is 0.60 μm, thediameter of the via hole 7 is 0.60 μm, the thickness of the secondinsulating film (i.e., the thickness of the upper and the lower portionsfrom level of the top of the first metallic wiring 5) is 0.50 μm, thewidth of the via hole 7 extending laterally from the first metallicwiring 5 is 0.30 μm, and the taper angles of the via hole 7 (i.e., 90and 75 degrees) are the same as those described above with respect tothe first embodiment.

As noted, when etching the first metallic wiring 5, a depositional gas,such as SlCl₄, is used as the etching gas to form the desired taper (θ₃)of approximately 80 degrees. At this time, etching of a bottom face ofthe via hole 7 extending laterally from the first metallic wiring 5 ismade difficult by a lie-lag effect as the over-etching proceeds.Therefore, as the bottom face becomes narrower, leakage between the viahole 7 and the silicon substrate 1 or the gate electrodes 3' can bereduced or avoided. Furthermore, in the second embodiment the percentageof over-etching of the bottom face can be greater than that of the firstembodiment, so as to further reduce or avoid leakage.

Next, referring to FIGS. 6 and 7, a third embodiment of the presentinvention will be described. In the third embodiment, the structureexcept the shape of the via hole 7 can be the same as that of the firstembodiment. Therefore, the same or similar elements are identified bythe same numbers and need not be discussed again.

In this third embodiment, as shown in FIGS. 6 and 7, the via hole istapered in two steps and is provided to decrease the contact resistancebetween the via hole 7 and the wiring 5 and to increase the operationalspeed of the device. A first predefined taper angle (θ₁), which ispreferably 88 degrees or more, is formed in an upper portion 7b of thevia hole 7 positioned over the top face 5a of the metallic wiring 5. Asecond predefined taper angle (θ₂), which is 88 degrees or less, isformed in a lower portion 7a of the via hole 7 under the top face 5a ofthe metallic wiring 5, which is an over-etched portion.

As seen in FIG. 7, as the first taper angle (θ₁) is formed in the upperportion 7b of the via hole 7, an area in which the via hole contactswith the first metallic wiring 5 is greater than in the case when thetaper angle (θ₁) is less than 88 degrees. Therefore, a contactresistance between the wirings when scaling down the semiconductordevice can be reduced, and higher operation speeds of the semiconductordevice can be achieved.

As the semiconductor device is scaled down further, the diameter of thevia hole can be 0.50 μm or less, the width of the first metallic wiring5 can be 0.50 μm, the diameter of the via hole 7 can be 0.50 μm, thetotal thickness of the second insulating film 6 can be 1.00 μm, thethickness of the upper portion of the second insulating film 6 over theupper face of the metallic wiring 5 can be 0.50 μm, the width of the viahole 7 extending laterally from the first metallic wiring 5 can be 0.20μm. If the taper angle (θ₁) is 80 degrees, the width of the area inwhich the via hole 7 contacts with the first metallic wiring 5 can be0.20 μm. On the other hand, if the taper angle (θ₁) is 88 degrees, thewidth of the area can be 0.29 μm. When scaling down, the difference of0.10 μm between areas of contact can affect the contact resistancesignificantly. Therefore, high speed operation of the semiconductordevice can be achieved by providing via holes having taper angles of 88degrees or more on the upper portion thereof.

The present invention also provides methods for manufacturing the abovedescribed semiconductor devices which will now be described. A firstmethod is as follows. As noted above, an end of a plasma etching cyclefor forming the via hole 7 is detected by monitoring a luminousintensity in a known etching process. To illustrate, after etching theupper portion of the second insulating film over the top face 5a of thefirst metallic wiring 5, the amount of etching material is decreased andthe end of the etching cycle is detected. To change the taper angle ofvia hole 7, an etching pressure is changed between before and after theend of the etching cycle. As a result, as shown in FIG. 6 and 7, thetaper angles θ₁ and θ₂ of the via hole 7 are achieved. Though thetypical pressure is different for the various methods and plasma etchingsystems utilized, when a typical pressure of 100 mTorr or more is used,the etching pressure is lowered to decrease the taper angle.

A second method is as follows. The end of the plasma etching cycle forforming the via hole 7 is similarly detected by monitoring the luminousintensity of the plasma. After etching the upper portion of the secondinsulating film 6 over the top face of the first metallic wiring 5, theamount of etched material is decreased and the end of the etching cycleis detected. A flow rate of an etching gas is changed between before andafter the end of the etching cycle to over-etch the via hole. As aresult, as seen in FIGS. 6 and 7, the taper angles θ₁ and θ₂ of the viahole 7 are achieved.

Usually when the silicon oxide film which is used as the insulating film6 is etched, a depositional gas such as CHF₃ is mixed with an etchinggas, such as HF₄ or C₂ F₆, and the silicon oxide film is etched by thegas mixture. When the proportion of the depositional gas is higher, thetaper angle is smaller.

A third method is as follows. As in the above methods, the end of theplasma etching cycle for forming the via hole 7 is detected bymonitoring the luminous intensity of the plasma. As in the abovemethods, after etching the upper portion the second insulating film 6over the top face the first metallic wiring 5, the amount of etchingmaterial is decreased and the end of the etching cycle is detected. Theetching before the end of the etching cycle is executed by a gasincluding hydrogen, fluorine or carbon such as CHF₃, and a gas notincluding oxygen such as CH₄ or Ar. The etching after the end of thatetching cycle (i.e., the over-etching cycle) is executed by a gasincluding hydrogen, fluorine or carbon, such as CHF₃, and a gasincluding oxygen such as O₂, CO or CO₂. In addition, after the end ofthat etching cycle, a gas not including oxygen such as CH₄ can be added.

As described above, the etching gas used is controllable and as aresult, the taper angles θ₁ and θ₂ of the via hole 7 are controlledrespectively, as seen in FIGS. 6 and 7. In the case of etching by a gasincluding oxygen, such as O₂, CO or CO₂, as compared to the case of theetching by a gas not including oxygen, the lie-lag effect in the formercase is greater than in the latter case. Therefore, as the etching speedin the bottom face of the via hole 7 of the former case is slower thanthe etching speed in the latter case, leakage between the via hole 7 andthe silicon substrate 1 or the gate electrodes 3' can be suppressedeffectively.

As described above, according to the first embodiment of the presentinvention, as the taper is formed in the lower portion of the via hole 7under the top face of the first metallic wiring 5, the via hole is keptfrom reaching the silicon substrate 1 or the gate electrodes 3' by alie-lag effect. As a result, shorting and/or leakage between the viahole 7 and the silicon substrate 1 or the gate electrodes 3' can besuppressed.

According to the second embodiment of the present invention, as thetaper is formed in the first metallic wiring 5, the area of the bottomface of the via hole 7 can be decreased further and the via hole can bekept from reaching the silicon substrate 1 or the gate electrodes 3' bythe lie-lag effect. As a result, shorting and/or leakage between the viahole 7 and the silicon substrate 1 or the gate electrodes 3' can besuppressed.

According to the third embodiment of the present invention, as thetaper, e.g., 88 degrees or more, is formed in the upper portion of thevia hole 7 above the top face of the first metallic wiring 5, the areain which the via hole contacts with the first metallic wiring isincreased as compared to the case when the taper angle is less than 88degrees. When scaling down the semiconductor device, the contactresistance between the via hole 7 and the first metallic wiring 5 can bereduced and a higher speed operation of the semiconductor device can beachieved.

Numerous modifications and variations of the present invention arepossible within the scope of the invention. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced in many ways different from those specificallydescribed herein.

What is claimed is:
 1. A semiconductor device comprising:a substrateformed of a semiconductor material; a first insulating film formed onthe substrate; a first metallic wiring formed at least partially on thefirst insulating film; a second insulating film formed on the firstinsulating film; a second metallic wiring formed at least partially onthe second insulating film; and a via hole for electrically connectingthe first metallic wiring and the second metallic wiring, the via holehaving a lower portion which extends to a level below a top face of thefirst metallic wiring, the lower portion being tapered so as to suppressleakage or shorting between the via hole and the substrate, wherein anupper portion of the via hole above a top face of the first metallicwiring has substantially vertical walls.
 2. The semiconductor device ofclaim 1, wherein an angle of the taper is 88 degrees or less.
 3. Thesemiconductor device of claim 1, wherein the first metallic wiring is atleast partially tapered and an angle of the taper of the first metallicwiring is 88 degrees or less.
 4. A semiconductor device comprising:asubstrate of a semiconductor material; a first insulating film formed onthe substrate; at least one gate electrode formed at least partiallywithin the first insulating film; a first metallic wiring formed atleast partially on the first insulating film; a second insulating filmformed on the first insulating film; a second metallic wiring formed atleast partially on the second insulating film; and a via hole forconnecting the first metallic wiring and the second metallic wiring, thevia hole having a lower portion which extends below a level of a topface of the first metallic wiring, the lower portion being tapered tokeep the via hole electrically insulated from the substrate and the atleast one gate electrode, wherein an upper portion of the via hole abovea top face of the first metallic wiring has substantially verticalwalls.